temp gauge goes down when heater turned on

[jocko 1.2]

tried searching but could find nothing definitive.

everything works as it should. 12 o clock needle position w/ac on or off. i get warm air when heater is on, however, the needle drops down by 1/4. no coolant leaks or overheating issues.

temp sensor?

faulty gauge?

heater core?

couldn't be thermostat/bleeding related as the car is not overheating right?

thanks in advance.

[CMG]

Either your heater core is a radiator or it's a faulty gauge or sensor is my guess. Blowing hot air will cool it some, but not that much.

[Hoad]

Well the heater core is a small radiator that interior air moves through to heat itself. I would not think that the needle should move that much though when you turn the heater on.

[jocko 1.2]

thank you gentlemen, keep the thoughts coming. i'm at a loss!

[polishracing95]

could be a sensor?.... i have never heard of such a problem

[Hoad]

I wouldn't think it's a sensor problem because turning on the heater has no relation to the sensor. Air bubbles the sensor can send the temp on the gauge up, but not down.(?) With the car not getting over halfway on the temp gauge, I can't say something is wrong. Unless the auxiliary (electric pusher) fan kicks on when the heater is on? I would still check the sensor. You wouldn't want to be overheating and not know it. Page 170-4 in the service manual.

[RRSperry]

Thermostat. It's sticking open... Check the water outlet temp with an IR thermometer.

[284Shooter]

Thermostat. It's sticking open... Check the water outlet temp with an IR thermometer.

+1.

Also, if you have the defroster on (no matter what temp you have set) it automaticlly runs the A/C, which runs the pusher fan. The recirc does the same.

Most thermostats are being built to stick open now. The guy who figured that design out deserves a Al Gore's Nobel prize and free gas for LIFE!

Chris

[Hoad]

Does it take a long time to get up to half way on the temp gauge?

[jocko 1.2]

Does it take a long time to get up to half way on the temp gauge?

not at all.

+1.

Also, if you have the defroster on (no matter what temp you have set) it automaticlly runs the A/C, which runs the pusher fan. The recirc does the same.

Most thermostats are being built to stick open now. The guy who figured that design out deserves a Al Gore's Nobel prize and free gas for LIFE!

Chris

i'll look into this. thanks again fellas.

[Hoad]

not at all.

Then I would not think that it is the thermostat stuck open because that's the the only problem that comes from that. When the car is at operating temperature, the thermostat is open, just like it would be if it was stuck open. So turning on the heater would have no effect on the thermostat, it would still stay at the position it was right before the heater was turned on. My guess is that it is an electronic problem. Jocko, does the electric fan turn on when the heater is turned on? Oh and has this only happened once, a few times, or all the time?

[Instanced]

could be a sensor?.... i have never heard of such a problem

This doesn't sound like a problem to me I wish my car did this... then i wouldn't overheat in traffic

[jocko 1.2]

Then I would not think that it is the thermostat stuck open because that's the the only problem that comes from that. When the car is at operating temperature, the thermostat is open, just like it would be if it was stuck open. So turning on the heater would have no effect on the thermostat, it would still stay at the position it was right before the heater was turned on. My guess is that it is an electronic problem. Jocko, does the electric fan turn on when the heater is turned on? Oh and has this only happened once, a few times, or all the time?

i don't think the fan turns on but i'll have to check w/the car @ idle sitting in the garage. car is a recent addition and i noticed said condition while driving it home from sacramento to socal. aside from this peculiar detail, the car runs beautifully. it's got over 200k mi. previous owner resurfaced head/installed head gasket some time last year.

This doesn't sound like a problem to me I wish my car did this... then i wouldn't overheat in traffic

you're right, things could be MUCH worse. i've encountered several e36 cooling issues over the span of 15 years and more than 2 dozen e36's. would like to know what's going on though to avoid anything potentially catostrophic.

[lesl]

Sounds like your coolant level dipped below the coolant sensor, which is located near the top of the head. You either have a lot of air in your system or you have a crack in the return pipe inside the coolant reservoir. Do you get the "coolant low" error? But when you check the reservoir, it looks fine?

[///m3neizel]

thermostat.

[jocko 1.2]

fan does not turn on when heater is on.

Then I would not think that it is the thermostat stuck open because that's the the only problem that comes from that. When the car is at operating temperature, the thermostat is open, just like it would be if it was stuck open. So turning on the heater would have no effect on the thermostat, it would still stay at the position it was right before the heater was turned on. My guess is that it is an electronic problem. Jocko, does the electric fan turn on when the heater is turned on? Oh and has this only happened once, a few times, or all the time?

a lot of air would cause overheating no?

no error message on obc.

Sounds like your coolant level dipped below the coolant sensor, which is located near the top of the head. You either have a lot of air in your system or you have a crack in the return pipe inside the coolant reservoir. Do you get the "coolant low" error? But when you check the reservoir, it looks fine?

thermostat.

prob change the t-stat just to run down a variable. it was changed about a year ago though.

[lesl]

I can see if the t-stat was stuck open, a sudden rush of cooler temp circulating from the heater core would drop the temp.
Do you have the classic cases of a stuck open t-stat? Long warm up times?
If the t-stat was stuck closed, it would overheat. You indicated that it was not overheating.

If you are not getting the coolant low messages, then you probably don't have a breach in the coolant syphoning system. It would take excessive air for the coolant to dip below the temp sensor located in the head, so that's probably not your issue.

Even after a coolant change, several driving trips should rid the air in the system to the reservoir. This is indicated by a lower coolant level in the reservoir. For grins you can check for air in the system anytime. When cool, take off the bleed cap and squeeze the top rad hose. Coolant should be visible. If engine is hot, loosening the bleed cap should start to leak fluid. If you get a lot of hissing instead, then you are pushing out air. If you continue to have air in your system, there is a problem somewhere.

Just some thoughts on this odd issue.

[jocko 1.2]

I can see if the t-stat was stuck open, a sudden rush of cooler temp circulating from the heater core would drop the temp.
Do you have the classic cases of a stuck open t-stat? Long warm up times?
If the t-stat was stuck closed, it would overheat. You indicated that it was not overheating.

If you are not getting the coolant low messages, then you probably don't have a breach in the coolant syphoning system. It would take excessive air for the coolant to dip below the temp sensor located in the head, so that's probably not your issue.

Even after a coolant change, several driving trips should rid the air in the system to the reservoir. This is indicated by a lower coolant level in the reservoir. For grins you can check for air in the system anytime. When cool, take off the bleed cap and squeeze the top rad hose. Coolant should be visible. If engine is hot, loosening the bleed cap should start to leak fluid. If you get a lot of hissing instead, then you are pushing out air. If you continue to have air in your system, there is a problem somewhere.

Just some thoughts on this odd issue.

very good, i'll need to allocate some tinker time over the weekend. thanks man.

[Hoad]

From my expereance air by the sensor would make it read overly hot. A leak would make it over heat becasue the system can't hold pressure.

[ososinsk]

i've encountered several e36 cooling issues over the span of 15 years and more than 2 dozen e36's.

You, sir, are a patient and dedicated man.

[jocko 1.2]

You, sir, are a patient and dedicated man.

still have fond memories of my first, a 92 325i and every other e36 incarnation since!

it's a bummer they never offered the e36 wagon stateside.

[Gugsbmw]

Sounds just like mine. Thermostat.

[Beamerdreamer07]

Jocko 1.2 did you ever figure out your problem with you losing coolant temp when the blower motor is on? I'm having the same problem. I know it's not the thermostat. Only thing I've thought was the water pump but dont wanna flop 200 for something that isn't broke. Thanks

[pbonsalb]

He may not be active 11 years later. Obviously, the heater core is a small radiator, and the blower motor is a small fan pulling heat out of it like your primary cooling fan does for the big radiator up front. The stock gauge is buffered and reads dead center from about 170 to about 220, so if your gauge is moving towards cold when you turn the heat on, the engine is operating on the cold end of the range. What temp thermostat are you using and is it working properly? The other possibility is a bad temp sender for the gauge.

[Beamerdreamer07]

He may not be active 11 years later. Obviously, the heater core is a small radiator, and the blower motor is a small fan pulling heat out of it like your primary cooling fan does for the big radiator up front. The stock gauge is buffered and reads dead center from about 170 to about 220, so if your gauge is moving towards cold when you turn the heat on, the engine is operating on the cold end of the range. What temp thermostat are you using and is it working properly? The other possibility is a bad temp sender for the gauge.

I'm not sure of the brand for the thermostat. I got it off of amazon. It did fix my original problem however. I'm viewing the coolant temp with my obd2 port scanner. As soon as the blower motor is turned on, even on the lowest setting, the temp starts to drop. I replaced the sensor as well.